This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 199 61 068.1, filed on Dec. 17, 1999, the entire disclosure of which is incorporated herein by reference.
1. Field of the Invention
The invention relates to a piezoelectric actuator system comprising at least two piezoelectric actuators mechanically arranged in series for producing an output motion or power.
2. Background Information
Piezoelectric actuators have the advantage of a high actuating precision and a fast reaction. Such actuators are components with a high electrical capacity whereby only part of the electrical energy supplied to the actuators is converted to mechanical energy. A large part of this energy is stored in the piezoelectric actuator functioning as a capacitor.
In a dynamic, repetitious or continuos operation of a piezoelectric actuator, considerable electrical power in the form of apparent power flows through the actuator. This apparent electrical power has to be supplied by the driver circuit of the actuator. In the case of periodic driving or energizing the piezoelectric actuator is electrically alternately charged and discharged, whereby electrical energy is cyclically supplied to and withdrawn from the piezoelectric actuator. In known driver circuits for periodic or repetitious driving of the piezoelectric actuator, the stored electrical energy is dissipated during the discharge phase or cycle in the driver or control circuit which has for example an ohmic resistance for the dissipating.
The German Patent Publication DE 197 39 594 C1 describes the use of driver circuits for actuator systems in which energy is dissipated during the discharge cycle. To achieve higher output forces and/or thermal compensation, two piezoelectric actuators are mechanically arranged in series and clamped against each other. In the known system the piezoelectric actuators are symmetrically driven in push-pull fashion so that with the stroke remaining the same, the output forces of the two piezoelectric actuators are added together. Thermal expansions of the piezoelectric actuators compensate each other in the known arrangement and a bias spring commonly used in such piezoelectric actuators is not required.
European Patent Publication EP 0 676 036 B1 describes trimorphic bending piezoelectric actuators with an energy dissipating driver circuit. A trimorphic bending actuator comprises a carrier or base plate with piezoelectric elements glued to both sides of the base plate which functions as one electrode. Both piezoelectric elements form electrical capacitors which are electrically driven reciprocally or in push-pull fashion.
In view of the above it is the aim of the invention to achieve the following objects singly or in combination:
to create a power-saving piezoelectric actuator system which operates periodically or continuously and is driven or energized in push-pull fashion;
to store and use energy, that is conventionally dissipated, in the reverse charge phase of twin piezoelectric actuators; and
to minimize the recharging or reverse charging energy in a piezoelectric actuator having at least two actuator elements.
According to the invention, there is provided a piezoelectric actuator system characterized by an amplifier bridge circuit including a first piezoelectric actuator forming a first quarter bridge branch, a second piezoelectric actuator forming a second quarter bridge branch, said first and second quarter bridge branches being connected in series with each other to form a first half bridge having a first junction point between said first and second piezoelectric actuators, a first electronic switch forming a third quarter bridge branch, a second electronic C1 switch forming a fourth quarter bridge branch, said third and fourth quarter bridge branches being connected in series with each other to form a second half bridge having a second junction point between said first and second electronic switches, said first and second half bridges being connected in parallel with each other to form a full bridge including a third junction point between said first and second half bridges to form a bridge power input and a normally grounded fourth junction point between said first and second half bridges opposite said power input, a choke connected to said first and second junction points, a power supply (U+) connected to said third junction point forming said bridge power input, and a control circuit connected to said first and second electronic switches for opening and closing said electronic switches to energize said first and second piezoelectric actuator""s.
The solution according to the invention provides the advantage that the electrical energy stored in the piezoelectric actuators is not dissipated in the driver circuit formed by the electronic switches and a control circuit of the actuator system. Instead the energy is recovered and used again with the aid of the choke. Furthermore, the above-mentioned advantages of an automatic thermal compensation are also achieved in the piezoelectric driver system according to the invention.